A typical fuel pumping system for an aero-engine comprises a low pressure (LP) pumping stage operable to draw fuel from a fuel tank, and supplying the fuel at boosted pressure to the inlet of a high pressure (HP) pumping stage. The LP pumping stage typically comprises a centrifugal impeller pump whilst the HP pumping stage typically comprises a positive displacement pump in the form of a twin pinion gear pump. The LP and HP stages are typically connected by a gear train to a common drive input, which facilitates a compact system and helps the LP and HP stages to operate synchronously. US 2010/024778 describes such a system.
The inter-stage flow between LP and HP pumping stages is typically used to cool engine lubrication oil in a fuel/oil heat exchanger. Accordingly, while the fuel in the LP pumping stage may typically be at a temperature of around 70° C., the fuel in the HP pumping stage may be at a temperature of around 140° C.
Not all the fuel exiting the HP pumping stage may be burnt in the engine. A substantial proportion may be recirculated back to the pumping system. For example, when the engine is operating at altitude where the engine burns little fuel, there may be significant fuel recirculation, raising the temperature of the fuel in the pumping system.
In addition, fuel pumping systems are typically compact, reduced mass designs constructed from highly conductive materials such as aluminium alloy. These can result in the LP pumping stage being exposed to high levels of heat transfer from the HP pumping stage. For example, heat produced by the rotating gears of the HP pumping stage can transfer into the LP pumping stage.
Consequently, when the engine is operating at altitude there can be a risk of flashing or vaporising of fuel at the inlet to the LP stage.